In lithography used for production of a semiconductor, formation of a finer pattern is increasingly demanded due to increase in the integration density, and, at present, lithography techniques with KrF excimer laser light (with a wave length of 248 nm) or ArF excimer laser light (with a wavelength of 193 nm) are used for mass production. Further, research and development are in progress also for lithography techniques using F2 excimer laser light, which has a shorter wavelength (a wavelength of 157 nm), EUV (extreme ultraviolet) and X-ray, which have still shorter wavelengths than those excimer lasers, and the electron beam.
With the progress of miniaturization of patterns as described above, a copolymer having a lower molecular weight has been increasingly used in recent years as a base component for resist used in lithography. This is because a copolymer having a smaller molecular size is thought to be preferably used in order to improve resolution and Line Edge Roughness (LER) (Patent Document 1). Actually, as for a copolymer used for ArF excimer laser, a copolymer having a weight-average molecular weight (hereinafter referred to as Mw) of about 10000 is mainly used for pattern size of not more than 90 nm, whereas a copolymer having a Mw of about 7000 has been increasingly used for pattern size of not more than 45 nm. Although a copolymer having a low molecular weight may have an undesired performance such as decrease in coating property, decrease in etching resistance, film loss at the time of immersion exposure or the like, it is thought that a most-advanced resist copolymer will tend to have a lower molecular weight in the future.
In these situations, a method for producing a copolymer having a low molecular weight, specifically a copolymer having a Mw of from 3000 to 6000 is not established. A most common method for producing a practically applied resist copolymer is the method in which several kinds of monomers are copolymerized in the form of solution under heat with a radical polymerization initiator such as an azo compound or the like to obtain a copolymer. In such a case, for reducing a molecular weight of a copolymer, a method in which the amount of used polymerization initiator is increased; a method in which polymerization is carried out in the presence of a sulfur compound or the like as a molecular weight modifier having a chain transfer effect, or the like is known (Patent Document 2).